There's a thread somewhere on TLS with Yale 250s of those who applied and got in. Can't seem to find it now though....from what I remember, a lot of people wrote about intellectual pursuits and academic topics. But there were also those who wrote about a personal talent or anecdote. I'm writing about a childhood nickname myself.

I think any topic is fine, but of course keeping the audience in mind (i.e. don't make it TMI). What's your diversity statement on? If you're submitting a diversity statement to Yale anyway, you might consider writing about another interesting aspect of yourself for the 250.